Your collarbone is first to harden and among the last to finish
The collarbone starts turning to bone in the fifth and sixth weeks after conception, before any other bone in the body. Yet it is one of the last to finish, not completing the process until somewhere between 21 and 25 years of age. In between, it becomes the bone people break more often than any other.
Its Latin name, clavicula, means little key, because it twists on its long axis like a key turning in a lock when the arm is raised out to the side. About 15 centimetres long and shaped like a stretched S, it is unique among the long bones in running sideways rather than up and down. One end meets the breastbone and the other meets the shoulder blade, so it works like a crane's strut, holding the shoulder out from the ribcage so the arm can swing through its full range and passing knocks from the arm into the central skeleton. It also shields the nerves and blood vessels heading into the arm.
Its shape varies more than that of most long bones. Men's are usually longer and larger, and the left is usually longer but weaker than the right. A small extra muscle linking it to the neck vertebrae shows up in only 2 to 3 per cent of people, and in a condition called cleidocranial dysostosis the collarbones can be partly or wholly missing. Falls onto the shoulder or an outstretched hand typically snap it where its two curves meet, its weakest point.
Doctors use an imaginary vertical line through the middle of the bone as a landmark for finding the heartbeat's apex or checking the liver.
Deep history explains its odd character. It began as part of the skull in primitive bony fish, sitting behind the gills, and it is still classed as a dermal bone. Early four-limbed animals lost the bones that tied the shoulder to the head, freeing a neck. Crocodilians and salamanders have no clavicles at all, turtles built theirs into the belly shell, and fast-running mammals often shrink or lose them to give the shoulder blade more freedom. In some theropod dinosaurs the pair fused into a furcula, the forerunner of the wishbone in birds.
Source: Clavicle
